1. Field of the Invention
The present invention relates to bulk absorbers having altered dielectric or magnetic properties, and more specifically, to bulk absorbers with predetermined concentration gradients.
2. Description of the Prior Art
Bulk absorbers are commonly used for absorbing radiation. A bulk absorber has varying particle concentrations throughout the absorber, which alter the dielectric or magnetic properties of the absorber. The particle concentrations can be designed to absorb target waves, depending on the application.
The prior art discloses absorbers have varying concentrations of dielectric or magnetic altering particles on a surface. These other absorbers include R-cards, R-film, and R-foam, as disclosed in U.S. Pat. Nos. 5,494,180 and 5,374,705, both of which are entitled xe2x80x9cHybrid Resistance Cards and Methods of Manufacturing Same,xe2x80x9d and co-pending U.S. patent applications entitled xe2x80x9cScreen Ink Printed Film Carrier and Methods of Making and Using Same from Electrical Field Modulationxe2x80x9d filed Dec. 10, 1997, and xe2x80x9cR-foam and Method of Manufacturing Samexe2x80x9d filed Mar. 25, 1998, all of which are expressly incorporated herein in their entireties.
The prior art also discloses fabricating bulk absorbers with a discontinuous concentration gradient of particles with dielectric or magnetic altering properties. The prior art bulk absorber is fabricated by laminating billets together. Each billet has a continuous concentration of dielectric or magnetic altering particles, resulting in a uniform dielectric or magnetic altering property gradient. The billets are laminated together with a bond layer in between each adjacent billet. As each billet is of a different concentration, the prior art bulk absorber has a step-wise concentration gradient of particles, and, as a result, a discontinuous dielectric or magnetic altering property gradient.
There are numerous disadvantages to the prior art bulk absorber with a discontinuous dielectric or magnetic altering property gradient. The discontinuities in the absorber, due to the step-wise changes in the dielectric or magnetic altering property gradient, cause reflection of the waves that are meant to be absorbed. Additionally, the bond layer in between the adjacent billets also causes reflection of the waves.
Therefore, what is needed is a bulk absorber fabricated such that wave reflection due to discontinuous dielectric or magnetic property gradients or bond layers is reduced or eliminated.
Accordingly, it is an objective of this invention to provide a bulk absorber having a continuous dielectric or magnetic altering property gradient and no bond layers, which results in reduced wave reflection.
In order to achieve the above and other objectives of the invention, a bulk absorber is provided with a body comprising a modified portion and particles dispersed throughout the modified portion in a substantially continuous concentration gradient. The particles have dielectric or magnetic altering properties. The particles may be carbon fibers, coated hollow microspheres, carbon black, carbon whiskers, or a combination thereof. The body may comprise foam materials or ceramic materials. The foam material may be syntactic or blown foam, and may be thermoplastic or thermoset.
More particularly, there is provided a bulk absorber radiation, which comprises a three-dimensional body comprised of a syntactic foam material and a plurality of magnetic or dielectric property-altering particles dispersed in a substantially continuous concentration of the three-dimensional body. The gradient extends along at least one dimension of the three-dimensional body (in one preferred embodiment, the body is a rectangular solid and the particle gradient extends along its depth, but can also extend along either or both of its height and its width), so that along the at least one dimension, the concentration of particles changes at a substantially continuous rate. The substantially continuous concentration gradient of property-altering particles results in a proportionally continuous rate of change of the altered property along the at least one dimension of the body.
A particularly important advantage of the present invention is that the inventive process for making the three-dimensional body enables its fabrication as a unified whole, meaning that, unlike the prior art, it need not be formed as a laminate comprising a plurality of billets, laminated together with bonding layers, wherein each billet has a distinctly different particle concentration, so that the process of laminating them together results in a stepwise change in particle concentration rather than the inventive continuous concentration gradient. In other words, the present invention resolves prior art problems concerning a predictable process for fabricating a bulk absorber having a continuous concentration gradient of property-altering particles along one or more dimensions of the bulk absorber body, so that it is no longer necessary to create a plurality of billets, each having a different uniform concentration of particles, and then laminating them together to create a stepwise particle concentration gradient. The inventive process and product is much better, and much less labor-intensive to make than the prior art approach.
In another aspect of the invention, there is provided a precursor for use in manufacturing a bulk absorber having altered dielectric or magnetic properties, comprising a syntactic foam comprised of an uncured resin. The precursor further comprises dielectric or magnetic property altering particles distributed in The resin in a substantially continuous concentration gradient therein. The gradient extends along at least one direction within the resin, so that along the at least one direction, the concentration of the particles changes at a substantially continuous rate, for providing a substantially continuous resistive taper in a bulk absorber to be molded using the precursor.
In another aspect of the invention a manufacturing system for fabricating the bulk absorber comprises delivery devices, control means, delivery means, positioning means, and forming means. The first and second bulk solids delivery devices produce first and second flows of absorber precursors through flow exits. The bulk solids delivery devices may be vibrational feeders. The control means varies the ratio of the flow rates of the first and second flows of absorber precursors. The control means may be any suitable device or control system for controlling the flow rates of the absorber precursors. The delivery means receives the first and second flows of absorber precursors from the flow exits and intermingles the flows to form a combined flow. The delivery means may comprise a generally horizontal conveyor belt having a discharge point. The positioning means deposits the combined flow in a predetermined pattern in a cavity to build a non-solidified item. The positioning means may comprise a translation means and/or a rotation means for changing the location of the cavity in a horizontal direction relative to the conveyor belt discharge point. The forming means is for solidifying the non-solidified item into the bulk absorber. In an aspect of the invention, the solidifying means sinters the non-solidified item in a sintering oven.
In another aspect of the invention, the process for manufacturing the bulk absorber has a first step of producing first and second flows of absorber precursors, wherein said first flow contains particles with dielectric or magnetic altering properties. Another step involves varying a ratio of flow rates of said first and second flows of absorber precursors. An additional step comprises intermingling the first and second flows of absorber precursors to form a combined flow. The manufacturing process also involves the step of depositing the combined flow in a predetermined pattern in a cavity to build a non-solidified item with a predetermined concentration gradient of particles with dielectric or magnetic altering properties. After the depositing step, the non-solidified bulk item is solidified into the bulk absorber.
In an aspect of the invention, the first and second flows of absorber precursors may be produced from vibrational feeders. The first and second flows are then intermingled by directing the flows to a generally horizontal conveyor belt such that the flows of particles overlap and discharged from a discharge point in a combined flow. The combined flow falls vertically into the cavity, with the cavity being positioned under the discharge point to adjust the cavity position relative to the discharge point such that the combined flow may falls in a predetermined pattern into the cavity. In another aspect of the invention, a bulk absorber is produced by the above-described process.
In an aspect of the invention, the magnetic or dielectric altering materials may be carbon fibers, coated hollow microspheres, carbon black, carbon whiskers, or a combination thereof. Further, the first and second flows of absorber precursors may comprise foam or ceramic material. The foam material may be syntactic or blown or may be thermoplastic or thermosetting.